Polyvinyl alcohol (hereinafter, sometimes abbreviated as “PVA”) is known to be a water-soluble synthetic polymer. A PVA exhibits particularly improved strength properties and film-forming properties in comparison with other synthetic polymers. A PVA is thus employed as a raw material for forming a film and a fiber; an additive for paper processing and fiber processing; an adhesive; a stabilizer for emulsion polymerization and suspension polymerization; a binder for an inorganic material; and the like. As described above, PVAs are valued in various applications.
A PVA is generally used as a dispersing agent for suspension polymerization of a vinyl chloride. In suspension polymerization, a vinyl compound dispersed in an aqueous medium is polymerized in the presence of an oil-soluble catalyst to give a particulate vinyl polymer. Here, in order to improve quality of a polymer obtained, a dispersing agent is added to an aqueous medium. Factors which dominantly influence quality of a vinyl polymer produced by suspension polymerization of a vinyl compound include a polymerization rate, a ratio of water to a vinyl compound (monomer), a polymerization temperature, a type and an amount of an oil-soluble catalyst, a model of a polymerization vessel, a stirring rate of the content in the polymerization vessel and the type of a dispersing agent. Among others, the type of a dispersing agent significantly influences quality of a vinyl polymer.
For a dispersing agent used for suspension polymerization of a vinyl compound, it is required (1) that an amount of coarse particles formed during polymerization is small and a vinyl polymer having a sharp particle size distribution is obtained, and (2) that a vinyl polymer with a high particle porosity and a high plasticizer absorbability is obtained. Conventionally, a cellulose derivative such as methylcellulose and carboxymethylcellulose, and a partially saponified PVA are used as a dispersing agent, alone or in combination. Such a dispersing agent cannot, however, satisfy the above requirements (1) and (2).
Non-patent Reference No. 1 (Kobunshi Kanko Kai, “Poval”, pp. 369 to 373 and pp. 411 to 415, 1984) has disclosed a PVA with a polymerization degree of 2000 and a saponification degree of 80 mol % and a PVA with a polymerization degree of 700 to 800 and a saponification degree of 70 mol %, as a dispersing agent used in suspension polymerization of vinyl chloride. Such a dispersing agent cannot, however, fully satisfy the above requirements (1) and (2).
Patent Reference No. 1 (JP 5-88251 B) has disclosed a dispersing agent consisting of a PVA wherein an average polymerization degree is 500 or more; a ratio of an weight-average polymerization degree Pw to a number-average polymerization degree Pn (Pw/Pn) is 3.0 or less; it has a structure comprising a carbonyl group and an adjacent vinylene group [—CO—(CH═CH—)2]; a 0.1% aqueous solution thereof has an absorbance of 0.3 or more and 0.15 or more at a wavelength of 280 nm and 320 nm, respectively; and a ratio of an absorbance (b) at a wavelength of 320 nm to an absorbance (a) at a wavelength of 280 nm [(b)/(a)] is 0.30 or more.
Patent Reference No. 2 (JP 5-105702 A) has disclosed a dispersing agent consisting of a PVA with a saponification degree of 75 to 85 mol %, an absorbance of 0.1 or more of a 0.1 wt % aqueous solution at a wavelength of 280 nm, a carboxyl-group content of 0.01 to 0.15 mol %, and a clouding point of 50° C. or higher of a 0.1 wt % aqueous solution.
Patent Reference No. 3 (JP 8-208724 A) has disclosed a dispersing agent for suspension polymerization consisting of a PVA having the above structure [—CO—(CH═CH—)2] in its molecule, an absorbance of 2.5 or more of a 1 wt % aqueous solution at a wavelength of 280 nm, an average polymerization degree of 500 or more, a saponification degree of 60 to 90 mol %, a ratio of a weight-average molecular weight Mw to a number-average molecular weight Mn (Mw/Mn) of 2.5 or less, a block character for a saponification degree of 0.45 or less, and a methanol-soluble content of 10 wt % or less.
Patent Reference No. 4 (JP 2007-063369 A) has disclosed a dispersing agent for suspension polymerization consisting of a PVA produced by copolymerizing a maleic acid derivative with a vinyl ester monomer followed by saponification to give a carboxyl-group containing PVA, then washing and drying the product, and then randomly introducing an ethylenic double bond starting from the carboxyl group into a main chain.
Patent Reference No. 5 (WO 2008/96727 A1) has disclosed a dispersing agent for suspension polymerization consisting of a PVA having an enone structure in a main chain in its molecule which is produced by copolymerizing carbon monooxide with a vinyl ester monomer followed by saponifying, washing and drying.
However, dispersing agents disclosed in Patent Reference Nos. 1 to 5 cannot always fully satisfy the requirements (1) and (2). Furthermore, a highly UV-absorbing PVA tends to be discolored, so that it may adversely affect hue of a molded vinyl polymer.